Nonthermal Plasma-Sustained CO₂ Methanation Over Ru-Based Multi-Metallic Catalysts

As a future low-carbon technology with minimum CO₂ emission, synthetic methane formation from CO₂, known as methanation or Sabatier reaction, is being highlighted (CO₂ + 4H₂ = CH₄ + 2H₂O; DH = -165 kJ/mol). Methanation is an exothermic reaction and the low-temperature condition is favored thermodynamically to achieve high CH₄ selectivity. Meantime, room temperature operation is obviously impossible kinetically: Catalyst temperature needs to increase up to 300-400 °C so that methanation reaction occurs at a high reaction rate while maintaining high selectivity. If catalyst activity is excellent, catalyst temperature reaches up to 500 C, creating a detrimental hotspot at the reactor inlet: high-performance catalyst would conflict with the low-temperature operation and high CH₄ selectivity and yield may not be realized. In order to optimize reaction conditions, high CH₄ selectivity, and yield, nonthermal plasma (NTP) was applied to heterogeneous catalytic reactions. NTP activates CO₂ and H₂ which promotes methanation at 100 C, while heat generation by methanation cooperatively promotes radical-enhance methanation at an elevated temperature of around 300 C. As a result, CH₄ yield achieved 100 % without an external heating source. NTP-catalyst interfacial reaction is the primary key to the reaction enhancement, while unique heat and mass transfer pray a unique role in the methanation reaction at high yield.<br/>This study focuses on the nonthermal plasma effect on CO₂ methanation over Ru-based multi-metallic catalyst using dielectric barrier discharge (DBD). To this end, CO₂ conversion behaviors in a packed-bed DBD reactor at 30 kPa with catalyst temperature between 150 °C and 400 °C were investigated. Two types of catalysts were studied: First, La(3wt%)-Ni(11wt%)/Al₂O₃ catalyst was studied as a control catalyst. Second, this catalyst was modified by adding Ru(1wt%) in order to promote hydrogen spillover which is the key step for CH₄ synthesis at high yield [1]. The total flow rate was 1200 cm³/min (at STP*) and dielectric barrier discharge (DBD) was applied at 30 W which corresponds to the specific energy input of 0.38 eV/molecule. H₂/CO₂ ratio was varied from 2 to 10.<br/>Ru-modified catalyst yielded CH₄ selectivity and CO₂ conversion of 100%, respectively at 300 °C: CH₄ yield reached the thermodynamic equilibrium. In contrast, without Ru modification, CH₄ yield was 30% with 60% CH₄ selectivity at 350 °C. Moreover, there is no reaction promotion by DBD without Ru at all [2]. Obviously, Ru promotes hydrogen spillover that promotes CO₂ conversion and CH₄ selectivity simultaneously. More importantly, the catalytic performance of Ru-modified catalyst is further improved by superposing DBD.<br/>The tentative reaction pathway is as follows. CO₂ and H₂ are dissociated on Ru and Ni respectively, while Ru also provides H adsorption sites that spill over hydrogen to nearby Ni sites. CO₂ is directly reduced to CO, followed by C or CH_xO formation and then hydrogenated to form CH₄. Additionally, CO is obtained from formate (HCOO) through prior generation of bicarbonate (HCO₃) while the Ru-support interface and Lanthanum (La) make contributions to this. Our previous study showed that La provides reactive cites which form carbonate species when CO₂ is activated by DBD [3]. Carbonate further reacts with surface hydrogen when Ru is co-exist nearby La and Ni. HAADF-STEM analysis support such mechanism because Ru, Ni, and La are well dispersed and overlapped on Al₂O₃ support. We are performing <i>in situ</i> infrared absorption spectroscopy which reveals a transient behavior of surface species under the presence of DBD. More detailed mechanistic insight is presented in the symposium.<br/>* STP: Standard Temperature and Pressure (25 °C and 101 kPa).<br/><br/>This work has been supported by JST CREST (JPMJCR19R3).<br/><br/>[1] S Saeidi et al, Prog Energ Combust, <b>85</b> (2021) 100905.<br/>[2] X Chen et al, <i>J CO₂ Util</i>, 54 (2021) 101771.<br/>[3] Z Sheng et al,<i> Phys Chem Chem Phys</i>, <b>22</b>(34) (2020) 19349.